The mass assembly of automotive and truck vehicles includes the filling of braking devices with brake fluid. This includes the step of using a vacuum pump to first remove air from the vehicle hydraulic system associated with the braking device. Next, pressurized brake fluid is allowed to flow into the hydraulic system. Finally, the hydraulic system and braking device are tested. This procedure is standard in the industry.
For the brakes to work properly, there cannot be bubbles in the hydraulic system. Some residual gas remains in the brake system after pumping that must be absorbed into the brake fluid. Brake fluid arrives at the assembly plant saturated with dissolved air. Consequently, if the as-received fluid is used without processing, it cannot dissolve the residual air, and bubbles end up in the hydraulic system. To eliminate bubbles, dissolved air is removed from the brake fluid before it is used by dripping the fluid through a vacuum chamber maintained at a pressure &lt;1 kPa. The deaerated fluid at the bottom of the chamber is pumped into a high pressure (about 100 psi) line and piped to the fluid-fill station. It is possible, however, for subtle equipment malfunctions to short circuit the air removal process or introduce air into the brake fluid. For example, if a pump has a bad seal, then air is injected into the high pressure line.
After the brake system has been filled with fluid, it is tested for leaks or air bubbles. Force is applied to the brake system's primary piston, and the piston's motion is monitored. A leak in the brake system causes the piston to move slowly after the initial compressive displacement, while a bubble affects the initial displacement. Even without bubbles, the initial displacement measured for a group of properly functioning vehicles can have a large amount of scatter. This makes it difficult to identify the extra displacement caused by bubbles as something new when it first appears. Consequently, bubble-related problems are not easily diagnosed by the initial displacement. Also, excessive initial piston displacement may be symptom of brake-system problems other than excessive air content. Consequently, the diagnosis of problems in the brake-processing equipment can represent a difficult endeavor, and an unambiguous measurement of air content in brake fluid would be desirable.
A malfunction that causes the assembly plant to produce vehicles with air bubbles in the brake lines is very serious. Because proper functioning of a vehicle's brakes is essential for safety, each vehicle produced with air in the brake lines must later have the air bled out before the vehicle can be shipped.
Dissolved gas in liquids is often detected in one of two ways: (1) the gas is phase separated into a bubble, the volume of which is measured, or (2) by an electrochemical method. As currently practiced to detect air in brake fluid, however, both techniques employ mercury, which makes their application questionable in a vehicle-assembly plant. Small quantities of a particular dissolved gas can also be detected by bubbling a carrier gas through a liquid and using standard analytical techniques to detect the species of interest in the carrier gas.
Volumetric methods to measure dissolved gas in liquids have been used in the past. A device is available "to determine free, entrained, and dissolved air in hydraulic and other fluids." The device consists of a transparent container and a manually operated vacuum pump that uses mercury as the working fluid. A sample of the fluid is placed in the container. The pressure on the fluid is reduced to near zero with the hand operated vacuum pump. The dissolved air forms bubbles, which rise to the top of the container. The change in the liquid level indicates how much dissolved air was present in the liquid.
Electrochemical methods to detect dissolved oxygen in liquids have been known. Because aqueous (water based) solutions are of the greatest practical importance, most electrochemical sensors have been developed for use in water. In particular, the Clark oxygen sensor, developed to measure dissolved oxygen in blood, is widely available. Another important technique is polarography with the dropping mercury electrode. This laboratory technique is presently used by brake fluid manufacturers to analyze samples of brake fluid for dissolved oxygen.
Polarography makes use of the potential difference that can be maintained over atomic dimensions at the interface between an electrode and an electrolyte. For each molecule that reacts, a known amount of charge passes through the circuit. Current is measured versus the applied potential difference. For small potential bias, the current increase is usually linear and moderate. As the potential bias is increased, the reaction rate and the corresponding current increase exponentially with the potential difference associated with the interface (i.e., the surface overpotential). As the potential difference is increased further, reaction resistance at the surface stops being the rate limiting step. All of the molecules that reach the surface react, which gives rise to a current that is limited by the rate at which fresh reactant is brought to the surface by convection and diffusion, a condition known as being transport limited. The great advantage of the dropping mercury electrode over other forms of polarography is that the working-electrode surface is continually renewed. A common problem with solid electrodes is that a film develops on the electrode that eventually influences the electrode behavior.
Thus, heretofore there has been a need for a device and method of reliably determining the amount of air or gas in brake fluid which overcomes the shortfalls of the prior art.